Include all headers regardless of depth

The previous hack for this was to move headers 3 levels deep in to
the top level include dir and headers 4 levels deep in to sys.
Obviously this was fairly limiting and missed two important headers.

  ./zfs/include/sys/fm/fs/zfs.h
  ./zfs/include/sys/fs/zfs.h

This patch updates the code to be smarter and installs all the headers
to the correct location regardless of depth.  Each header will have its
leading 3 path args stripped (i.e. ./zfs/include/) and replaced with
the correct install destination.  All path information past the first
three levels will be preserved.

module/Makefile.in

@@ -23,12 +23,8 @@ modules_install:
 	# Install the required headers in to the kernel source
 	destname=zfs-@ZFS_META_VERSION@/@LINUX_VERSION@; \
 	instdest=$(DESTDIR)/@prefix@/src/$$destname; \
-	(mkdir -p $$instdest && \
-	 find . -mindepth 3 -maxdepth 3 -name '*.h' | \
-	 xargs cp -t $$instdest) || exit 1; \
-	(mkdir -p $$instdest/sys && \
-	 find . -mindepth 4 -maxdepth 4 -name '*.h' | \
-	 xargs cp -t $$instdest/sys) || exit 1;
+	(find . -mindepth 3 -name '*.h' | xargs -Ihdr sh -c \
+	 "DEST=hdr && install -D hdr $$instdest/\$${DEST#*/*/*/}") || exit 1
 
 distdir:
 	distfiles=`find . -name '*.c' -o -name '*.h'`; \